Experimental and numerical study is performed on subsonic hydrogen jet diffusion flame formed from the vertical circular nozzle. Emphasis is placed on the effect of the cavity height formed at the fuel injection nozzle tip on suppression of the flame lift-off. It is found that (i) an increase in the cavity height triggers and enhances a vacuum pressure, (ii) the air from the surroundings is transported naturally into the cavity to replenish the air entrained and consumed by the jet flame, and (iii) the vacuum pressure results in the mitigation of flame liftoff propensity.
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